During the 1950's, many studies on abundant nutritive components of beans and cultivation of beans were actively carried out, and isoflavone contained in soybeans was recently received attention as a functional substance.
Soybean isoflavone is called a vegetable estrogen since it plays a role as an estrogen, a female sex hormone. It has a preventive effect on hypertension, osteoporosis, breast cancer, prostate cancer and a menopausal disorder. That is, soybean isoflavone inhibits a release of calcium from bones of a woman to inhibit a decrease in bone density, which results in a preventive effect on osteoporosis. While breast cancer cells require estrogens, genistein contained in soybeans shows an anti-estrogenic activity in women before the menopause and reduces activities of estrogens associated with breast cancer via hormones and cell metabolism, which results in a preventive effect on breast cancer. On the other hand, a death rate of Japanese people suffering from prostate cancer is very low compared to that of occidentals. It has been reported that the said difference was resulted from different intake of foods made of soybeans. Also, it has been reported from an animal experiment that intake of soybeans inhibited the advance of symptoms of prostate cancer and growth of prostate tumour. Since production of estrogens, a female sex hormone is suppressed during the menopause, women may suffer from a headache, arthralgia and unstableness of body temperature. However, isoflavone reduces symptoms of a menopausal disorder since it shows a weak estrogenic activity.
Isoflavones composed of genistein, glycitein, daidzein, and their glucosides and derivatives are contained in soybeans. While most of them exist in the form of glucosides such as genistin and daidzein, some of them exist in the form of aglucones such as genistein and daidzein due to hydrolysis of carbohydrate residues by .beta.-glucosidase during microbial fermentation and enzyme reaction. The said isoflavone glucosides and aglucones are absorbed within the intestines. Since an absorption rate of aglucones converted by fermentation or enzyme reaction is much higher than that of glucosides, it has been known that isoflavone aglucone contained in fermented foods has an advantage in a view of utility value, compared with isoflavone contained in non-fermented foods.
In general, it has been revealed that aglucone is contained in a ratio of 10% and less in non-fermented foods and in a ratio of 70-80% in fermented foods such as soybean paste and fermented soybeans. Micro-organisms known to hydrolyse isoflavone glucosides up to now are Bifidobacterium longum, Lactobacillus bulgaricus, Aspergillus niger and Sacchoromyces, and have a common feature that they produce .beta.-glucosidase.
It has been reported that isoflavone in the form of isoflavone aglucone prepared by removing glucoside from isoflavone glycoside was hydrolyzed during fermentation using Lactobacillus debrueckii (see: Korean Journal of Food Science 3(1):185-195(1999)). In addition, U.S. Pat. No. 5,554,519 discloses a process for preparing genistein isoflavone which comprises fermentation using Saccharopolyspora erythraea and extraction of genistein isoflavone with organic solvent in a condition of pH 8-11, and a Japanese patent laid-open publication No. 60-199396 discloses a process for preparing isoflavone derivatives, daidzein and tectorigenin which comprises fermentation of Streptoverticillium Sp. K-251. A Japanese patent laid-open publication No. 50-160483 discloses that Aspergillus niger is cultured aerobically in a medium including potato starch, glucose and soybean for 5 days at 27.degree. C. to produce isoflavone active in vivo, and a Japanese patent laid-open publication No. 50-35393 discloses that Actinomyces roseolus is cultured in a medium including soybean, glucose and starch in a neutral condition to produce isoflavone inhibiting catechol-O-methyl transferase activities. In processes using enzymes, esterase, pectinase and cellulase as well as .beta.-glucosidase have been used to hydrolyse glucosides to convert into isoflavone aglucone. For example, WO 9510512 discloses a process for preparing vegetable whey abundant in isoflavone aglucone by hydrolysing isoflavone glucosides with .beta.-glucosidase, esterase or acid. The said enzymes show different conversion ratios according to substrates.